1. Field of the Invention
This invention relates to a video camera having an automatic focusing function.
2. Description of the Prior Art
Previously, a method which uses a high frequency component of a video signal generated in a video camera (hereinafter called a "video signal system") has been well-known as an automatic focusing apparatus (auto-focus system) for use in video cameras. This system is one of the most effective systems in that no generation of parallax results theoretically and the focusing can be achieved with high accuracy.
For example, an auto-focus system disclosed in U.S. Pat. Application Ser. No. 272, 898 filed on Nov. 18, 1899 is equipped with a frequency filter for extracting frequency components of horizontal and vertical directions of an image plane to switch the characteristic of the frequency filter according to an information of an object, thus being capable of achieving the focusing operation highly definitely.
An auto-focus system for use in video cameras is required to execute operations always at a high level of accuracy in response to various types of objects with a wide range of illuminance and contrast properties. That is, a signal processing unit in the auto-focus system is required to have a wide dynamic range. In order to construct the signal processing unit with a digital circuit, it is required that the number of bits of an Analog-Digital Converter (hereinafter called an A/D Converter) to be placed at the initial stage of the signal processing unit is made large and the number of bits of a signal processing circuit to be successively placed thereto is made equal to or larger than that of the A/D Converter. This makes the structure of the signal processing unit extremely complex.